What I would do with this unit is one of the following approaches:
Aproach A: It is a boat, so use some modification of Naos and ships rules from the book. I do not read them recently, so I could not be more specific, but I remember the rules were very interesting.
Aproach B: Marienburg landship is a tower with the special hability of moving.
So It is a tower, could be destroy as all the tower with two damage of artillery. It can be assault with ladders or with siege towers. Can hold a unit with the status of fortified... The tower rules.
About the moving I would use the rule of Dog of war phalanx restriction, and limit his movement to 20cm as arillery.
Adds on: could have boiling wax cauldruns (maybe two one of each side of the boat), and maybe could have some cannons like the boat section says it have a ship.
IMHO If I would use in my group that unit I would do not mess with stat lines for a unit like that, as a machine is difficult to measure, and a consensus could be difficult to reach. Also IMHO I think it is needed to clearly split the idea of fluff "the wonder of the age" to the profile stat "a balance unit to have fun in the game as an add on the list and not the central piece of the game (besides you would like an special scenario that this unit will be central piece of the game"
Hope it helps,
Anyway very nice models you both have